1. Field of the Invention
The invention relates generally to aeroengine fuel circuits. It relates more specifically to a device for reducing the fuel delivery needed at the outlet of a high pressure pump of a circuit when the engine speed is below a predetermined threshold, for example in an in-flight re-ignition configuration. The invention also relates to a method for reducing the delivery of fuel used, at a given engine speed or under given flight conditions.
2. Technological Background
In an aeroengine, fuel is supplied to fuel injectors, at which the fuel is burnt, to provide the energy for propulsion and for engine auxiliaries. The fuel is also used as a coolant in heat exchangers, and to operate a plurality of hydraulic servovalves. For example, jet pipes and other variable-geometry members are operated by hydraulic rams which are driven by a supply of the fuel under pressure.
The servovalves are normally supplied from a central servovalve feed unit whose pressure must remain constant and whose delivery must be capable of being varied in accordance with servovalve requirements.
The circuit supplying the injectors, the exchangers and the central servovalve feed unit comprises a low pressure pump which raises fuel coming from the aircraft tanks at an initial pressure Pf to a pressure Pl, and a high pressure pump which further raises the fuel to a pressure higher than the pressure needed by the fuel injectors and by the central servovalve feed unit.
Unused fuel is recycled. It is well known that the proportion of fuel recycled compared to the fuel used must not be excessive, since the fuel is heated by its passage through the high pressure pump and is therefore less suitable as a cooling liquid. The high pressure pump may be a pump whose delivery is determined solely by its rotational speed. This speed is itself a function of the engine speed. The pump may also be a pump whose delivery is a function not only of its rotational speed, but also of another parameter which can be so controlled that a pump running at a given speed can provide different delivery rates.
The dimensions of the high pressure pump are calculated so that the pump can provide the necessary delivery regardless of the engine speed. It has been found that the conditions which are the most unfavorable, relative to the dimensions of the pump, are the conditions at the moment of in-flight re-ignition.
During re-ignition in-flight, the fuel supply needed by the engine fuel injectors is obviously low, but the other requirements of the engine, particularly those of the variable geometry members, remain more or less constant.
As a result, the high pressure pump is dimensioned to satisfy conditions which are encountered only very exceptionally during the life of an engine. It then follows that, in order to counter a rare eventuality, the high pressure pump is oversized for practically all other flight conditions.
This means that the high pressure pump is heavy compared with the delivery that is required most of the time. It also means that there is significant recycling of unused fuel, that is to say return of a significant fraction of the fuel to the low pressure side of the pump through a regulating (bypass) valve, whereby the unused fuel is heated up by the rise in pressure it has experienced. As a result, the hotter fuel is less able to perform its cooling function, and the heat exchangers need to be sized to take this into account.
Thus, the pump, the heat exchanger and, for delivery supply reasons, other parts of the fuel circuit, are oversized in order to meet requirements in highly exceptional flight conditions, namely in-flight re-ignition.
By comparison with the state of the art just described, the object of the invention is to reduce the size of the high pressure pump, and therefore the delivery provided by the pump under all flight speed conditions. This results in a reduction in the size of associated parts of the fuel circuit and, in particular, in the size of the heat exchangers, whose coolant is the fuel. On the whole, the mass and therefore the cost of a circuit comprising the invention, are reduced.
According to the invention, in a fuel supply circuit of an aircraft engine including fuel injectors and a plurality of servovalves, a low pressure pump for raising the pressure of fuel from a tank from a pressure Pf to a pressure Pl, and a high pressure pump for further raising the pressure of the fuel to a relatively high pressure, the high pressure fuel supplying said plurality of servovalves, and supplying said fuel injectors through a pressure regulator, there is provided a device connected downstream of said high pressure pump and upstream of said plurality of servovalves for controlling the supply of fuel from said high pressure pump to said plurality of servovalves, said device having an open position in which said high pressure pump is in communication with said plurality of servovalves and a closed position in which said plurality of servovalves is not supplied by said high pressure pump, said device receiving a command to move to said closed position in response to the pressure of said fuel dropping below a predetermined value or in response to the rotational speed of said engine dropping below a predetermined value.
Cutting the fuel supply to at least some of the servovalves, and therefore some of the variable geometry members, in particular those which it is known will not be used during a phase of re-ignition of the engine in-flight, reduces the fuel delivery needed.
Thus, the delivery available is directed as a matter of priority to the fuel injectors. The high pressure pump may therefore be dimensioned for a lower delivery, thus reducing the mass and the volume of the pump. Since the pump is of a smaller size, the delivery is also lower, and the volume of fuel that has been heated up by the rise in pressure and is recycled is thus reduced. As a result, the fuel used as coolant in the heat exchangers is colder, and the volume of said exchangers can be reduced.
Thus, despite the addition of the device for controlling the supply of fuel to the servovalves, the overall mass and the volume of the fuel circuit are reduced.
In a preferred embodiment making it possible to further reduce the size of the high pressure pump, for engine speeds below the threshold rotational speed, the fuel injector supply pressure is reduced. Thus, the pressure of the fuel delivered by the high pressure pump is also lower and, in consequence, the delivery of this pump, for the same rotational speed, is higher.
In general, the fuel injector supply pressure is regulated by a pressurizing valve. This valve has an opening for the outlet of fuel toward the fuel injectors. The cross section of this outlet opening is a function of the position of a spool. The axial position of the spool within the pressurizing valve is a function of the forces exerted on the spool in the two opposite directions along the valve axis. One or more springs exert a force on the spool in one axial direction, and the fuel pressure received on one or more pistons of the spool exerts a force in the opposite axial direction.
In general, the law governing the movement of the spool, and therefore the law governing the variation of the cross section of the outlet opening of the valve, is such that the pressure of the fuel increases with the delivery consumed by the fuel injectors.
According to a preferred embodiment of the invention, said pressure regulator operates according to a first or a second pressurization law, said second law being used when the engine speed is below a predetermined value, the pressure of fuel delivered to said fuel injectors as a result of said second law being lower than the pressure which would have resulted from said first law for the same rotational speed of the engine.
One means of producing this modified pressurization law is to change the nature of a fuel pressure applied to a piston of the spool.
According to an alternative embodiment of the invention, when the engine speed drops below the threshold value, the supply of high pressure fuel to the bearings of the high pressure pump can be reduced, possibly to the point of cutting it off altogether. When the high pressure is cut off, the bearing 14 receives only the low pressure Pl.
Since the rotational speed of the high pressure pump is low at low engine speeds, it is sufficient for the fuel supply to the pump bearings to be at low pressure, or at a reduced high pressure delivery. It should also be noted that the durations of use corresponding to in-flight re-ignition are very short, which means that the risk of damaging the bearings is practically zero.